1. Field of the Invention
The present invention relates to hearing aids. The invention further relates to hearing aid systems and to a method for processing audio signals. More specifically the invention relates to hearing aid systems capable of processing signals from more than one type of signal source, such as a microphone in combination with any one of a radio wave receiver, an audio-input device, a telecoil receiver, an optical receiver (e.g. infrared) and the like. The invention, in a further aspect, relates to a method for enhancing the signal-to-noise ratio (SNR) in a composite hearing aid system.
2. The Prior Art
Hearing aids having more than one input are well known. Hearing aids having inputs for different types of signals, herein designated composite hearing aids, also exist. Particularly well known examples comprise hearing aids with a microphone input and with a telecoil input. DE-A-3032311 discloses a radio receiver accessory adapted for plug-in connection to a hearing aid in order to provide a radio reception capability. The receiver is powered by the hearing aid battery. U.S. Pat. No. 5,734,976 discloses a miniature radio receiver adapted for connection to a hearing aid fitted with an additional loop antenna. A switch permits changing the balance between microphone input and radio input.
U.S. Pat. No. 6,307,945 provides a personal hearing aid system. The hearing aid system interfaces with existing hearing aids using the “T” facility (i.e. a telecoil capability). The system comprises a microphone, an FM radio transmitter connected to the microphone, a receiver unit for receiving a signal from the transmitter unit, and a hearing aid with a “T” facility. The receiver unit connects to an induction loop, and the hearing aid receives the signal from the induction loop and transmits an audio signal.
U.S. Pat. No. 6,516,075 shows a hearing enhancement system for co-operation with a conventional hearing aid used in “T”-switch mode, including a microphone and an induction loop. The induction loop is worn around the body of a speaking person. The induction loop generates an electromagnetic signal that may propagate some distance away from the speaking person to be picked up by a telecoil-enabled hearing aid.
U.S. Pat. No. 5,615,229 provides a short range wireless communications system employing a belt worn receiver coupled via a cord or cable to a loop which is worn under the clothing of the hearing aid user. The hearing aid in turn has an inductive pick up coil for picking up the loop signal. The receiver may include RF receiver circuitry to pick up and convert an RF signal to an audio frequency electrical signal.
In a composite system, the transmitter is typically positioned near a distant sound source that is of interest to the hearing-impaired individual. The delivery of information from the transmitter to the receiver, connected to the hearing-impaired individual's hearing aid, will thus permit the audibility of the distant sound sources. The main use for a composite hearing aid system is in situations where the preferred acoustic source, e.g. an orator, has a remote, but well known, location and where additional use of the hearing aid microphones is advantageous. For the hearing-impaired, these situations include educational settings, meetings, public presentations, church sermons and the like. In these situations a wireless receiver is beneficial in order to achieve an appropriate S/N ratio and an increased speech intelligibility for the hearing aid user.
Nevertheless, using a wireless receiver with a hearing aid without using the hearing aid microphones also exposes some inherent problems in use. One problem is the reduced ability to pick up wanted sounds other than those being fed directly into the transmitter, e.g. comments from parts of the audience outside the range of the transmitter microphone. This can impair the ability to participate in, for instance, an educational setting, as the inclination to ask any questions is modest if one cannot hear his or her own voice.
The hearing aid user may have a wireless receiver for both hearing aids (left and right) or for just one of them. When using wireless receivers on both hearing aids, the signals reproduced by the two receivers can be presumed to be identical and mutually in phase, i.e. they are perceived as a diotic signal.
In research dealing with determining perception of signals in noise, both the noise source and the desired signal source are often controlled to a great extent. The noise level and the balance between the noise and the desired signal determine the conditions under which experiments are carried out. The noise source usually masks the signal in some way, and is therefore denoted a masker. Different properties like intelligibility or hearing threshold level may be examined during such experiments, including binaural conditions.
A diotic signal may be a stimulus presented in the same way to both ears, M0S0, where M denotes a masker and S denotes a desired signal of the combined stimulus. This condition should be distinguished from the monotic condition, MmSm, a stimulus presented to one ear only, and from the dichotic condition, where the stimulus is presented differently to the two ears, e.g. M0Sπ, M0Sm, MπS0, etc. This is explained in further detail in the following, where S denotes the signal and M denotes the masker.
If a signal is presented binaurally in a homophasic condition (the same signal is presented in an identical form to both ears), this signal can be denoted S0, where the suffix 0 indicates the lack of phase difference between the signals presented to both ears. Likewise, a signal presented 180° out of phase to one ear when compared to the other ear can be denoted Sπ, where the suffix π denotes the antiphasic relationship between the two signals.
In the dichotic conditions, one of the two stimuli (i.e. the tone) is presented differently to the two ears, binaurally (e.g. SπS0, where the speech is presented in phase binaurally while the masker is presented 180° out-of-phase binaurally).
A well-known method for improving perceived SNR exploits a psychoacoustic phenomenon known as the binaural masking level difference (BMLD). Listening tests have revealed that a difference in masking level can improve the ability to detect a tone presented to the listener in competing noise. The BMLD is evaluated where tones are presented to both ears at the same time that a masking or competing noise is being delivered binaurally (Licklider, 1948). See table 1. The listener is tested under two conditions, a homophasic and an antiphasic condition. In the homophasic condition the speech or tones are presented either monotic to one ear, MmSm, or diotic to both ears in phase, M0S0.
TABLE 1Interaural condition compared to MmSmMLD (masking level difference)Monotic, dioticMmSm, M0S00 dBDichoticMπSm6 dBDichoticM0Sm9 dBDichoticMπS013 dB DichoticM0Sπ15 dB 
When the signal and masker are presented in this antiphasic fashion, a maximal release from masking is obtained, i.e. the listener is able to comprehend a tone level that would otherwise have been buried by the masker. The difference in thresholds between the homophasic and antiphasic condition reveals the BMLD. Green and Yost (Handbook of Sensory Psychology, Springer-Verlag, 1975, pp 461-465) have demonstrated a BMLD effect of up to 15 dB in a population of normal listeners (Table 1). The BMLD, as shown in table 1, is limited to deal with detection of pure tones in unmodulated broadband noise only, but are incorporated to explain the principles behind the invention.
Currently, the masking level difference may be observed in systems where only one of two hearing aids is equipped with a wireless receiver, and where the HA microphones are active, “ON”, corresponding to the dichotic condition M0Sm, thus giving a theoretical benefit of 9 dB if pure tones are used for the signal.
Green and Yost verified these values with white noise with a spectrum density level of 60 dB as the masker and a low-frequency sinusoid, e.g. 500 Hz, presented intermittently to the listener at brief durations of approximately 10-100 ms, as the signal. The conclusions drawn from the experiments are that the BMLD is never negative, but, for some binaural conditions, may be zero dB, i.e. no improvement.
A more practical approach may be taken by applying a different type of measurement, known as the binaural intelligibility level difference, or BILD. This test is based on the fact that the recognition of speech can be measured by presenting nonsense, one-syllable words, denoted logatomes, to a listener at varying sound pressure levels to determine the degree of syllabic recognition. This is measured as the percentage of syllables in a spoken sentence that are perceived correctly. The syllabic intelligibility level is defined as the sound pressure level of speech in connection with which a given degree, say, 50%, of syllabic intelligibility is attained. (Blauert et. al., Spatial Hearing, The MIT Press, 1974.)
In a real-life situation, even a modest improvement in SNR from a BMLD or a BILD may provide a major enhancement of the intelligibility of speech in noisy conditions. See table 2. One example of a situation where speech and masking noise are present is that of an educational setting. In this situation, the teacher is positioned in the front end of the room and there may be instances of noise from other students or from the environment that make it difficult, especially for hearing-impaired individuals, to hear what is being said by the teacher. For hearing-impaired listeners, the use of a composite system is often preferred in these situations in order to permit the delivery of acoustic characteristics of distant sound sources, such as the teacher's voice, to the ear.
TABLE 2Interfering noiseBILD, MπS0White noise, 75 dB7.2 dBModulated white noise fm = 4 Hz, m = 62%5.5 dB1 speaking voice4.3 dB1 speaking voice + white noise5.7 dB1 speaking voice + modulated white noise5.2 dB2 speaking voices9.0 dB2 speaking voices + white noise6.4 dB2 speaking voices + modulated white noise6.6 dB
The use of a composite system will thus improve the perceived SNR and facilitate the comprehension of the teacher's voice. However, in order for the hearing-impaired individual to monitor his/her own voice and the immediate acoustic environment, the hearing aid microphones are usually activated in the composite system together with the transmitter microphone, and this combination has a negative influence on the S/N ratio when compared to the wireless receiver on its own.
However, a moderate release from masking may be obtained in a composite system where the hearing aid microphones are activated, but where a wireless receiver is connected to only one of the two hearing aids. This corresponds to the M0Sm condition in table 1. This approach combines the advantages of a desirable SNR and monitoring of one's own voice. Also, this approach in providing composite systems is common practice by practising audiologists today, partly due to economical considerations.